The present invention relates to a vehicle steering apparatus, and more specifically to a vehicle steering apparatus provided with a variable transmission ratio device and a steering force assist device.
In recent years, steering control systems have been proposed which are provided with an active steering function. The steering control systems determine the steering characteristic of a vehicle by detecting vehicle-condition parameters such as the vehicle velocity and the yaw rate. Based on the determination result, the steering control systems control the steering angle (tire angle) of steered wheels to control the yaw moment of the vehicle.
For example, the vehicle steering apparatus disclosed in Japanese Laid-Open Patent Publication No. 2005-297719 is provided with a variable transmission ratio device as a device having an active steering function. The variable transmission ratio device changes the transmission ratio (gear ratio) between the steering wheel and the steered wheels. The gear ratio is changed by adding a second steering angle (ACT angle) of the steered wheels based on the activation of the motor to a first steering angle of the steered wheels based on a steering operation. When the vehicle is in an oversteer state, the variable transmission ratio device changes the ACT angle to control the tire angle in the direction opposite to the direction of the yaw moment (countersteering direction). In contrast, when the vehicle is in an understeer state, the variable transmission ratio device changes the transmission ratio such that the amount of change of the tire angle relative to steering operation of the steering wheel is reduced.
Also, the steering apparatus includes an electric power steering apparatus, which applies an assist force to a steering system using a motor as a driving source. The power assist control performed by the electric power steering apparatus is executed in cooperation with the active steering control. More specifically, when the vehicle is in the oversteer state, the electric power steering apparatus applies an assist force that encourages the driver to countersteer. In contrast, when the vehicle is in the understeer state, the electric power steering apparatus applies an assist force that suppresses increase of the steering wheel turning angle. Accordingly, the attitude of the vehicle is quickly stabilized.
However, in the above-mentioned steering apparatus, when the variable transmission ratio device is actuated, a torque (motor reaction force) in the direction opposite to the direction of the change of the ACT angle acts on the steering wheel. When the driver feels the motor reaction force as torque fluctuation, steering feel is deteriorated. In the active steering control, during counter control when in particular the change and the changing speed of the ACT angle is great, the influence of the motor reaction force tends to become further significant. In this point, there is room for improvement in the steering apparatus.